1. Field of the Invention
The present invention relates to a medical nebulizer and in particular to a medical nebulizer for providing a nebulized physiologically active liquid into an inspiration gas flow of a mechanical breathing aid.
2. Description of the Prior Art
Medical nebulizers are often used to deliver a metered dose of a physiologically active liquid into an inspiration gas stream for inhalation by a recipient. They generally operate to generate liquid droplets which form an aerosol with the inspiration gas. In other circumstances medical nebulizers may be used to inject water droplets into an inspiration gas stream to provide gas with a suitable moisture content to a recipient, this is particularly useful where the inspiration gas stream is provided by a mechanical breathing aid such as a respirator, ventilator or anaesthetic delivery system.
The term xe2x80x9cmedical liquidxe2x80x9d as used herein means any liquid, regardless of whether it is physiologically active, which is to be supplied to the airways of a recipient.
A known medical nebulizer is described in PCT Application WO 95/01137 and is a hand held device which operates to eject droplets of a medical liquid into a passing air stream (inspiration gas stream) which is generated by a recipient""s inhalation through a mouthpiece. This known device has a reservoir for the medical liquid which is connected to a capillary nozzle via a pump which operates directly on liquid within the capillary to eject liquid droplets through the nozzle and into an inspiration gas flow conduit. The pump is a bubble jet or piezoelectric pump, both of which are pulsed to eject a droplet through the nozzle with each pulse. A control unit is also provided to regulate the dose based on the number of pulses provided to the liquid by the pump, for example by regulating the time that a drive signal of known frequency is applied to the piezoelectric element of the pump.
A problem with this device is that the dosage is dependent on the pulse frequency. This limits the device to the delivery of relatively small doses unless a number of nozzle and pump arrangements are employed. This would increase the overall size of the device. Moreover when the piezoelectric pump is used the supply of a pulse to the piezoelectric crystal can stimulate the crystal to produce a pulse train of typically 5 to 6 pulses and lead to an inaccurate dose being provided. Furthermore, the pump is necessarily relatively small as it must act only on liquid within the capillary. The pumping power of such a pump tends to be limited so that at small capillary bore sizes flow resistance can become a problem, limiting the minimum droplet size to typically, microliters.
An object of the present invention is to provide a medical nebulizer which overcomes the pumping problems associated with the above-described known device.
The above object is achieved in accordance with the principles of the present invention in a medical nebulizer having a reservoir containing a medical liquid and a capillary nozzle having a first end communicating with the reservoir for receiving the medical liquid therefrom, and a second end connectable to an inspiration gas flow path in communication with a patient. A pump supplies the medical liquid from the reservoir through the capillary nozzle in a continuous flow, and a regulator regulates the delivery of the liquid to the nozzle so as to supply a predetermined amount of the medical liquid thereto. A stimulator is mechanically connected to the second end of the capillary nozzle and is operated to vibrate the second end of the capillary nozzle to stimulate droplet formation, so that droplets of the medical liquid are supplied into the inspiration gas flow.
By arranging for the pump to supply a continuous stream of liquid to be nebulized with the aid of a stimulator for vibrating the nozzle a metered dose of medical liquid can be provided in droplet form without the need to monitor the number of droplets.
Moreover, by using a pump which does not act directly on liquid within the capillary a more powerful and less expensive pump, for example a syringe pump, may be used. This enables the capillary bore size to be reduced over known nebulizer and picoliter sized droplets may be supplied. The finer droplet formation allows a more homogeneous vapor to be formed in the inspiration gas stream and a more efficient uptake of the medical agent by the patient.
The reservoir and the nozzle may be releasably couplable. In this way the reservoir may be readily disposed of, replaced or removed for sterilizing as necessary. This also provides an administrative benefit in that an estimate of the amount delivered can be readily obtained from a count of the used reservoir cartridges, enabling for example, stocks to be timely replenished. The pump may be operated continuously and suitable valving provided, such as micro-valves which have particularly well defined cut on and off characteristics, to limit the supply of liquid in order to deliver the predetermined dose from the nebulizer. In this manner inaccuracies in the delivered dose due to the start up and shut down characteristics of the pump can be alleviated.
In order to co-ordinate the delivery of the liquid dose with an inspiration of a recipient, which may be either before, during or after an inspiration, a flow meter can be provided in the flow path of the inspiration gas and adapted to control the delivery dependent on a sensed gas flow.